Wireless (cellular) communication service providers customarily supply wireless communication capability to (mobile) subscribers located within a geographic area, through the use of a relatively limited number of communication channels. In order to optimize coverage within the geographical area of interest, the service provider typically subdivides the area into a cluster or multiple clusters of base stations. In addition, in order to minimize interference from adjacent or nearby cells, the service provider may employ some form of frequency reallocation (or reuse) scheme, such as that described in the U.S. Pat. No. 4,144,496, as a non-limiting example.
In such a spatially distributed or `cluster` network architecture, a fixed number of sectors (i) are served by a cluster of (k) base stations. This has the effect of subdividing the number of available channels N by the product of i and k, namely by (i*k). Unfortunately, with today's expanding traffic, particularly in densely populated urban areas, service providers face the eventuality of running out of channels to meet demand.
One solution is to construct more base stations and reduce power levels--which is both hardware intensive and expensive. Another scheme is to reuse channels in time (TDMA) or in frequency (CDMA). Other approaches, such as described in the above-referenced patent, include dynamic allocation of frequencies or channels to accommodate channel demand. Initially, the relatively poor efficiency of frequency allocation schemes was not a significant problem as the demand was small and the number of available channels was more than adequate. However, as demand increased, new channel assignment and frequency reuse strategies were developed.
Such schemes have included sectorization of cells to minimize interference, and dynamic allocation or `borrowing` of channels from other cells with a cluster, to meet unbalanced demand within the cluster. A new and promising approach is to spatially separate channels using switched or steered antenna beams. The overall objective of any strategy is to maximize the number of channels available, subject to an acceptable carrier (C) to interference (I) ratio, with the current industry standard being a figure of merit (or C/I ratio) of 18 dB.
Sectorization is a technique that uses fixed beams formed by directional antenna (phased) arrays installed at the base stations to divide the cell into an integral number of smaller cells. This technique serves to reduce interference to the base station, by attenuating channel interference to those mobile subscribers who are not located in that sector's beam. It also reduces interference to the mobile subscriber, by attenuating channel interference from base stations transmitting in a direction that is predominately away from the location of the mobile subscriber. However, as the number of sectors increases, the number of channels per sector necessarily decreases, thereby reducing the figure of merit. Ideally, at the time of system installation, there would be no sectorization, which would greatly increase system capacity.
Regardless of the channel allocation mechanism employed, whenever a mobile subscriber moves from one cell to another, it is necessary to change the frequency channel used to conduct communications with the base station in the `old` cell from which the mobile transceiver is departing to a new frequency channel used to conduct communications with the base station in the `new` cell which the mobile transceiver is entering.
Techniques using steered beam antennas have unique problems accomplishing this handoff between cells. In particular, the `new` cell has the problem of where to point its narrowbeam antenna. The mobile subscriber is waiting on transmission from the new base station to transmit on the new frequency. If the new base station points the beam in the wrong direction, then the mobile subscriber sees no signal, does not synchronize and does not transmit. After the elapse of a prescribed period of time with no communication, the call will be dropped. The problem then is for the new base station to determine the correct beam to the mobile subscriber.
One mechanism for performing such frequency channel reuse/reallocation (or hand-off from the previous base station to the new base station) is described in the U.S. Patent to Forssen et al, U.S. Pat. No. 5,615,409. This scheme involves the base station using an `intermediate` channel to determine the direction of the mobile transceiver relative to it. It then assigns the mobile transceiver to an available narrowbeam channel. Because this technique requires what could otherwise be used for a regular communication channel be employed as an intermediate construct channel to determine the direction of the mobile transceiver, it necessarily reduces the number of available precious resources (channels).